FK506-binding protein 51 (FKBP5), which influences HPA axis activity via glucocorticoid receptors, is supposed to play an important role in the regulation of negative feedback and glucocorticoid resistance.

By fusing a destabilizing domain of the FK506-binding protein to essential cytomegalovirus proteins, we generated virus mutants in which amounts of fusion proteins and viral growth can be regulated by the synthetic ligand shield-1.

Here we show that mitochondrial FK506-binding protein 38 (FKBP38), unlike FKBP12, binds to and inhibits calcineurin in the absence of the immunosuppressant FK506, suggesting that FKBP38 is an inherent inhibitor of this phosphatase.